BigRep PET-CF Filament is a chopped-carbon-fiber-reinforced polyethylene terephthalate feedstock supplied in 2.85 mm-diameter monofilament form for large-format material extrusion. The supplier designates the grade as a rigid-tooling material positioned between unfilled PETG and polyamide 6/66 in its portfolio. The carbon fiber reinforcement increases tensile and flexural modulus, reduces thermal expansion in the print plane, and lowers moisture absorption compared with short-fiber reinforced nylon, while the polyethylene terephthalate matrix provides lower-warp processing behaviour than many semicrystalline materials. The filament is specified for use on large-frame fused filament fabrication machines with heated beds and is available in spool formats compatible with BigRep and third-party systems that accept 2.85 mm feedstock. The supplier datasheet lists specific gravity in the range of 1.30–1.36 under ISO 1183-1:2019, depending on fiber fraction.

Compounding of the carbon-filled PET is carried out on co-rotating twin-screw extruders with barrel length-to-diameter ratios in the range of 44:1–52:1. The carbon fiber is introduced through a downstream side stuffer after the PET resin has been melted, which limits fiber breakage and reduces the thermal history of the fiber sizing. Melt temperature at the die is held near 260–270 °C to avoid excessive PET chain scission; screw elements downstream of fiber addition are configured for distributive rather than high-shear mixing. Published data for this specific configuration is limited, but the resulting filament shows pronounced orientation-dependent mechanical properties when printed. Supplier literature reports that XY tensile values exceed Z-direction tensile values by a factor of approximately 1.5–2.0.

What Distinguishes This Grade From Unfilled PETG and Short-Fiber Reinforced Nylon?

Compared with unfilled PETG, the carbon fiber raises elastic modulus and lowers elongation at break. Published supplier data for BigRep PET-CF show tensile modulus in the range of 5.0–6.0 GPa for XY-oriented printed specimens tested under ISO 527-2:2012, whereas typical unfilled PETG grades are reported at 1.8–2.1 GPa. The corresponding trade-off is a reduction in strain at break from roughly 10–20 % for unfilled PETG to 2–4 % for the carbon-filled grade. Against polyamide 6 or 66, PET-CF exhibits lower equilibrium moisture absorption, typically below 0.5 % at 23 °C and 50 % RH; polyamide 6 can absorb more than 2 % under the same conditions. The lower moisture uptake reduces post-print dimensional change in humid environments but does not eliminate the need for drying, because the polyester backbone is susceptible to hydrolytic chain scission at processing temperatures.

Pre-drying at 65 °C for 4–6 h in a desiccant dryer with a supply-air dew point at or below −40 °C is specified before extrusion. In high-humidity environments above 60 % RH, drying time is extended to 8 h or the filament is kept in a heated hopper dryer during the build. Moisture levels above 0.02–0.04 % by weight in PET cause hydrolytic chain scission at melt temperature, leading to reduced interlayer weld strength and surface splay. On production-scale large-format machines, the failure mode is observed as delamination at sharp geometry changes after initial layers have cooled; this is frequently misattributed to bed adhesion but is more closely linked to moisture-induced reduction of melt strength. A vacuum drying oven at 80 °C for 8–10 h is an alternative when desiccant dryers are unavailable, but static-air ovens do not remove moisture from the core of a full spool at the same rate.

Unopened spools are stored at 20–30 °C and 50 % RH or lower. If a spool has been left open for more than 48 h at 60 % RH, pre-drying is mandatory even if the filament was previously dried. Partial spools should be returned to a sealed container with desiccant, and desiccant packs are recharged at 120 °C for 2 h. Incoming material is checked with a two-axis laser micrometer at intervals along the first 50 m of each spool. Ovality above 0.05 mm can cause inconsistent feeding through a 0.6 mm nozzle and is sufficient to require spool rejection or reconditioning.

When the carbon-filled melt reaches a large-format hot end above 240 °C

Extrusion temperature settings are specified between 240 °C and 260 °C, with a heated bed at 60–80 °C. The hot end should use a hardened steel, ruby, or silicon carbide nozzle of at least 0.6 mm, because chopped carbon fiber abrades brass and copper-alloy orifices. Orifice wear becomes measurable as an increase in extrusion width after 3–5 kg of throughput, depending on fiber length distribution and nozzle alloy; an enlarged orifice alters extrusion width and reduces dimensional accuracy. Direct-drive extruders with short melt zones are preferred over long Bowden paths. Retraction distance is kept below 3 mm and retraction speed below 30 mm/s to reduce the risk of grinding and clogging. Print speed for 0.6 mm nozzles is commonly set in the range of 30–60 mm/s; higher speeds increase melt pressure and can cause extruder skip. Layer height is typically 0.2–0.3 mm for large-format parts, with 0.4 mm used for faster infill only when visual surface quality is not critical. At processing temperature, the carbon-filled melt has higher thermal conductivity than unfilled PET, which accelerates solidification in cold zones and can produce layer-to-layer under-fusion if the layer time is too long. The recommended layer time is kept below 60 s for large tools; for very large layers, extrusion temperature is increased to 260 °C only after hot end thermistor calibration is verified.

First-layer adhesion on glass-reinforced build plates is achieved at a bed temperature of 60–80 °C with a polyetherimide film or PET-specific adhesive. A first-layer height of 0.2 mm and first-layer extrusion width set to 120–150 % of the nozzle diameter reduce the incidence of corner lift. On production-scale machines, release failures on first layers are more common when the bed temperature falls below 60 °C or when the build plate is cleaned with solvent residues containing silicones. When using a thin film adhesive, the surface is replaced or reconditioned after approximately 20–30 print cycles to maintain consistent peel strength.

Large-area prints above 500 mm in the longest axis are more sensitive to curl and corner lifting than smaller builds. The carbon fiber reduces linear thermal expansion to roughly 30–50 × 10⁻⁶ K⁻¹ in the print plane, compared with 60–80 × 10⁻⁶ K⁻¹ for unfilled polyester. Despite this, residual stress from differential cooling remains; on open-frame machines, edge lifting is controlled by maintaining bed temperature at 80 °C, applying a PET-specific adhesion layer, and using a brim of at least 10 mm. An enclosed build chamber or heated frame is recommended when the longest dimension exceeds 400 mm, but published data for this specific configuration is limited. In production-scale fixtures, flatness over a 500 mm × 500 mm tooling plate is typically measured with a granite surface plate and dial indicator; deviations greater than 0.5 mm are addressed by stress-relief annealing at 100 °C for 2 h on a flat support.

Mechanical property differences appear primarily in flexural loading.

Under ISO 178:2019 three-point flexural testing, XY-oriented BigRep PET-CF specimens show higher flexural modulus and lower strain at failure than unfilled PETG. Supplier datasheet values list flexural modulus at approximately 4.5–5.5 GPa and flexural strength at approximately 85–95 MPa. Tensile testing under ISO 527-2:2012 reports tensile strength near 60–65 MPa and tensile modulus near 5.0–6.0 GPa. Heat deflection temperature under 0.45 MPa load is listed in the range of 70–80 °C according to ISO 75-2:2013. These values are sensitive to print orientation: Z-direction tensile strength is lower than XY and often falls below 50 MPa in large-layer-height prints. The material therefore should be oriented in a part so that service loads run along the printing plane, not across layer interfaces.

Chemical resistance of the PET matrix is generally adequate for aliphatic hydrocarbons, dilute acids, and many automotive fluids at room temperature, but the material is not recommended for continuous exposure to strong alkaline solutions or hot concentrated acids. Stress cracking may occur when printed parts are placed under mechanical load while exposed to ester-containing solvents or certain glycols. Because the carbon fiber filler can create localized galvanic coupling, bonded metal inserts may corrode in humid environments unless the insert is isolated with a non-conductive barrier.

The main operational boundary for BigRep PET-CF is its low ductility and high notch sensitivity. The material is not suitable for snap-fit hinges, impact shields, or components requiring strain above 3–4 %. It is not specified for continuous service above its HDT under mechanical load, and direct food-contact use is not recommended unless the printed part is sealed with a certified food-contact coating because layer crevices can retain organic material. The carbon fiber filler increases tool wear during post-processing; carbide-tipped drills and saw blades are specified. The supplier safety data sheet indicates compliance with REACH 1907/2006 and RoHS 2011/65/EU, but specific migration data for printed parts in food-contact service is not provided. Long-term creep, fatigue, and UV stability data for large-format printed PET-CF parts is limited in supplier literature.